Mozilla said that a malicious program inserted adware code into a Firefox plugin that has been downloaded thousands of times over the past three months.

Firefox developers have now scanned all of their plugins. The Vietnamese language pack is the only one that had this kind of code, she said.

The open-source browser maker does not know how many people were infected with the adware, but the plugin was downloaded more than 1,200 times in the past week and has been downloaded 16,667 times since November.

"Everyone who downloaded the most recent Vietnamese language pack since February 18, 2008 got an infected copy," Snyder wrote.

"Mozilla does virus scans at upload time but the virus scanner did not catch this issue until several months after the upload."

Mozilla is now going to add additional scans of its software to prevent this kind of thing from happening in the future, she said.

"I think it (happened) just because the author's local network was infected with the virus, so it modified HTML files," wrote developer Hai-Nam Nguyen. "The infected code just display(s) annoying banner but it can't propagate."

Mozilla missed the code during its initial scan because antivirus vendors had not yet added detection for Xorer into their products, Snyder said in an interview. Antivirus vendor Panda Security first detected Xorer on February 28, 10 days after the infected plugin was published.

On Wednesday afternoon, the web page for the plugin was off-line as Mozilla scrambled to come up with a new, adware-free version of the language pack. In the meantime, users of the software should disable the plugin, Snyder said.

Xorer added a script to the Vietnamese language pack's HTML files that would have taken Firefox users to adware servers as they were surfing the Internet, Snyder said.

Snyder did not know exactly how the adware code was added, but she said that this kind of problem could affect any software provider - open source or not. "In most software development environments the developers aren't kept in a dark cave," she said. "They browse the web or take those laptops to a coffee shop "

"It's just a fact of life," she added.

Other vendors have been hit with similar problems. In late 2006 Apple shipped Video iPods that contained the RavMonE.exe virus. And late last year, retailer Best Buy shipped digital picture frames that contained malware.

Although some might say Mozilla's incident underscores the risks of open-source software development, this type of issue could crop up at a company like Microsoft too, said Eric Schultze, chief technology officer at Shavlik Technologies. "Most products that ship today include HTML files in them," he said via instant message. "Any one of them could suffer from this."

Mozilla was already doing the right thing scanning their code before upload, Schultze said. "But it shows the need to have tight security on developer systems."